A Reinterpretation of the Imidazolate Au(I) Cyclic Trinuclear Compounds Reactivity with Iodine and Methyl Iodide with the Perspective of the Inverted Ligand Field Theory

Coinage metal cyclic trinuclear compounds (CTCs) are an emerging class of metal coordination compounds that are valuable for many fine optoelectronic applications, even though the reactivity dependence by the different bridging ligands remains somewhat unclear. In this work, to furnish some hints to unravel the effect of substituents on the chemistry of Au(I) CTCs made of a specific class of bridging ligand, we have considered two imidazolate Au(I) CTCs and the effect of different substituents on the pyrrolic N atoms relative to classic metal oxidations with I2 or by probing electrophilic additions. Experimental suggestions depict a thin borderline between the addition of MeI to the N-methyl or N-benzyl imidazolyl CTCs, which afford the oxidized CTC in the former and the ring opening of the CTC and the formation of carbene species in the latter. Moreover, the reactions with iodine yield to the oxidation of the metal centers for the former and just of a metal center in the latter, even in molar excess of iodine. The analysis of the bond distances in the X-ray crystal structures of the oxidized highlights that Au(III)-C and Au(III)-N bonds are longer than observed for Au(I)–C and Au(I)–N bonds, as formally not expected for Au(III) centers. Computational studies converge on the attribution of these discrepancies to an additional case of inverted ligand field (ILF), which solves the question with a new interpretation of the Au(I)–ligand bonding in the oxidized CTCs, which furnishes a new interpretation of the Au(I)-ligand bonding in the oxidized CTCs, opening a discussion about addition/oxidation reactions. Finally, the theoretical studies outputs depict energy profiles that are compatible with the experimental results obtained in the reaction of the two CTCs toward the addition of I2, MeI, and HCl.


IR spectra
Crystals of compounds 1 and 2 are simultaneously formed as needles and platelets, respectively. Manual separation of the crystals was performed, and the IR spectra were recorded. Figure S1. Overlapped IR spectra in the range 4000-400 cm -1 of compound 1 (red lines, needles) and compound 2 (blue line, platelets) Figure S2. Overlapped IR spectra in the range 700-200 cm -1 of compound 1 (red line, needles) and 2 (blue line, platelets)

NMR spectra
Crystals of compounds 1 and 2 are simultaneously formed as needles and platelets, respectively.
Manual separation of the crystals was performed and the 1 H and 13 C NMR spectra were recorded in DMSO-d 6 . Both compounds are sparingly soluble in DMSO-d 6 . Compound 1 is less soluble than compound 2. The NMR spectra consist of several peaks but none of the presents can be attributed to the starting CTC Me . The solution of compound 1 in DMSO-d 6 displays signals for the imidazole protons at 7.74 and 7.40 ppm, and 125.27 and 122.99 ppm, respectively, attributed to compound 1 but, predominantly, the signals due to compound 2 were observed (see figure S2-S5). It is likely that the fully iodized compound 1, upon the difficult dissolution, loses iodine to give compound 2.
Moreover the overnight acquisition for the 13       Inset images are enlargements of the chemical ranges indicated by the arrows. The attribution of the Me-Au-I at 1.78 ppm was made according to data reported in the literature. 1 Figure S9. 13 C NMR spectrum in CDCl3 of compound 3, revealing the presence of CTC Me and free MeI (29 ppm) in addition the compound 3, with methyl bound to Au at 20 ppm. Upon long accumulation, additional species are formed too. Figure S10. Overlapped IR spectra in the range 4000 -600 cm -1 of the CTC Me (red line) and compound 3 (blue line). Although a good overlap of the bands is mostly observed, additional absorptions are observed at 1154 cm -1 (CH3 deformation) and 721 cm -1 (CH3 rocking) in the spectrum of compound 3, likely due to the deformation and rocking modes of the Methyl of the Me-Au-I moiety. The attribution was made according to data reported in the literature. 2 Figure S11. 1 H NMR spectrum of compounds 4 and 5 recorded in DMSO-d 6 .
Figure S12. 13 C NMR spectrum recorded in acetone -d 6 for compounds 4 and 5. The peaks at 28 and 206 ppm are due to DMSO, the peak at 77 is due to CHCl3, the peak at 54 ppm is due to CH2Cl2.  iodination follows similarly with a gain of -7.1 kcal mol -1 followed by +10.4 kcal mol -1 cost for the separation of the I3and a further gain of -17.4 kcal mol -1 occurs for the achievement of the fully iodinated species, 1. Scheme S1 summarizes the overall free energy pathway associated with the reactivity of CTC Me with di-iodine up to the final species 1.
Scheme S1. Free Energy pathway for the reaction between the CTC Me and di-iodine molecules up to the final complete iodinated species 1.

Computational studies on the reaction of HCl with CTC Me or CTC Bz
The reactivity between the original CTCs with methyl iodide highlighted the not innocent behavior of the imidazole ligand and their ability to catch a methyl cation. Thus, we wonder if such a discrepancy is also operative for the reaction with the hydrochloric acid or not. Once again, the investigation of direct involvement in the reaction is excluded in both cases while the analysis revealed again that the incoming substrate reacts with the aromatic system of the imidazolyl ring. The computational analysis perfectly mirrors the experimental results since in both cases the formation of carbene and bis-carbene moieties is observed. Figure S16 shows the Transition State for the reaction of starting compound with benzyl, Figure S16a, and with methyl substituents, Figure S16b.